A great many methods have previously been proposed for recovering oil from oil shale, nearly all of which involve some form of pyrolytic eduction. For a variety of economic reasons, none of these methods has yet proven competitive with the production of oil from petroleum or other fossil sources. In general, the principal overall difficulty involved in shale oil eduction resides in recovering essentially all of the hydrocarbonaceous material from the shale without resorting to prohibitively expensive methods. Since shale rock usually contains only about 20-80 gallons of oil per ton, it is a practical necessity to recover at least about 80 to 90% of this oil and to do so in the most economical fashion. It is not essential that all of this oil be recovered as a liquid product. Combustible gas products can also be economically utilized in the eduction process itself or in any other way where a source of heat is required. Nonetheless, the overall objective remains to recover the maximum possible energy values at minimum expense.
Perhaps the most widely used basic concept in oil shale retorting involves countercurrently contacting a stream of crushed oil shale with a stream of preheated eduction gas. In this manner a temperature gradient is set up in a moving shale bed which includes a hot eduction zone near the gas inlet to the retort and an oil condensation, shale preheating zone near the shale inlet end of the retort.
Two methods are commonly used, a solids-downflow mode of operation and a solids upflow mode of operation. In the solids-upflow, gas-downflow method, oil shale is fed upwardly through a vertical retort by means of a reciprocating piston. The upwardly moving shale bed continuously exchanges heat with a downflowing high specific heat, hydrocarbonaceous recycle gas introduced into the top of the retort at a temperature in the range of 950.degree. to 1200.degree. F. In the upper section of the retort (the pryolysis zone), the hot recycle gas educes hydrogen and hydrocarbonaceous vapors from the shale. In the lower section of the retort, the oil shale is preheated to pyrolysis temperature by exchanging heat with the mixture of recycle gases and educed product vapors. The product vapors are continually swept away from the hot pyrolysis zone and the heavier hydrocarbons therein condense in the lower portion of the retort. These heavy hydrocarbons are collected at the bottom of the retort as product oil. The remaining uncondensed gas is passed out of the retort through external condensing or demisting means to obtain more product oil. The remaining oil-free gases are then utilized as recycle gas to carry heat into the shale bed as above described and as fuel gas to preheat the recycle gas up to the above specified temperatures.
The product oil is recovered from the retort and further processed to remove solids therefrom. This generally is accomplished by what is referred to as deashing. As its name implies, deashing is accomplished in a deasher typically comprising an electrostatic coalescer which separates the oil from the retort into two fractions, a substantially pure product oil fraction and a sludge fraction containing entrained oil. Attempts have been made to remove the remaining entrained oil using centrifuges and the like. However, such attempts have met with only marginal success, removing only 10 to 20 percent of the entrained oil.